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Anal. Chem. 1986, 58, 824-827
C
0
n L
0.6
0
0.3
L 350
I
I
I
I
4 00
450
5 00
550
wavelength /
n rn
Figure 2. Absorption spectra of the colored product of acetaminophen in water (a) and in 40% acetone (d) and of aspirin in water (b) and in 4 0 % acetone (c).
trans acetylate 4-minophenol. The only interfering substance is acetaminophen and it should be predetermined. Registry No. Aspirin, 50-78-2; 4-aminophenol, 123-30-8; acetaminophen, 103-90-2.
LITERATURE CITED (1) Florey, K. I n "Analytical Profiles of Drug Substances"; Florey, K., Ed.; Academic Press: New York, 1979; Vol. 8, p 1,: (2) Connors, K. A.; Amidon, G. L.; Kennon, L. Chemical Stability of Pharmaceuticals", Wiley: New York, 1979; p 151. (3) Kelley, C. A. J . Pharm. Sci. 1970, 59, 1053. (4) Bundgaard, H. Arch. Pharm. Chem., Sci. Ed. 1976, 4 , 103. (5) Baum, R . G.; Cantwell, F. F. J . Pharm. Sci. 1978, 67, 1066.
(6) Kirchhoefer, R. D.; Reepmeyer, J. C.; Juhl, W. E. J . Pharm. Sci. 1980, 69,550. (7) Taguchi, V. Y.; Cotton, M. L.; Yates, C. H.; Miliar, J. F. J . Pharm. Sci. 1981, 70,64. (8) Mroso, P. V.; Li Wan Po, A.; Irwin, W. J. J . Pharm. Sci. 1982. 7 1 , 1096. (9) Reepmeyer, J. C. J . Pharm. Sci. 1983, 72, 322. (10) Walash, M. I.; Agarwal, S. P.; Martin, M. I.Can. J . Pharm. Sci. 1972, 7 , 123. (11) Belal, S. F.; Elsayed, M. A. H.; Elwalily, A.; Abdine, H. Analyst(London) 1979, 104, 919. (12) Boudeville. P.; Burgot, J. L.; Chauvel, Y. Analusis 1983, 11. 406. (13) Harris, P. A.; Riegelman, S. J . Pharm. Sci. 1967, 56, 713. (14) Nickerly, J. G. Anal. Chem. 1984, 36,2248. (15) Tam, Y. K.; Au, D. S. L.; Abbott, F. S. J . Chromafogr. 1979, 1974, 239. (16) Bateh, R. P.; Winefordner, J. D. J . Pharm. Biomed. Anal. 1983, 1 , 113. (17) Ascione, P. P.; Chrekrian, J. P. J . Pharm. Sci. 1975, 6 4 , 1029. (18) Gupta, V. D. J . Pharm. Sci. 1980, 60, 110. (19) Jalal, I.M.; Sa'sa, S. I.Talanta 1984, 3 1 , 1015. (20) Bevitt, R. N.; Mather, J. R.; Shaman, D. C. Analyst (London) 1984, 109, 1327. (21) Kirchhoefer, R. D.; Juhl, W. E. J . Pharm. Sci. 1980, 69, 548. (22) Galante, R. N.; Visalli, A. J.; Grim, W. M. J . Pharm. Sci. 1984, 73, 195. (23) "US. Pharmacopeia XX"; Mack Printing Co.: Easton, PA. (24) Cunningham, C. G.; Barkan, S. J . Assoc. Off. Anal. Chem. 1973, 56,667. (25) Cunningham, C. G.; Barkan, S. J . Assoc. Off. Anal. Chem. 1974, 57,728. (26) Verma, K. K.; Gulati, A. K. Anal. Chem. 1980, 52, 2336. (27) Verma, K. K.; Gupta, A. K. Anal. Chem. 1982, 5 4 , 249. (28) Verma, K. K.; Gulati, A. K. Anal. Chem. 1982, 5 4 . 2550. (29) Verma, K. K.; Jain, A.; Rawat, R . J . Assoc. Off. Anal. Chem. 1984, 67,262. (30) Verma, K. K.; Gulati, A. K.; Palod. S.;Tyagi, P. Analyst (London) 1984, 109, 735. (31) Verma, K. K.; Tyagi, P.; Gulati, A. K . J . Pharm. Biomed. Anal., in press. (32) Fieser, L. F.; Fieser, M. "Reagents for Organic Synthesis"; Wiiey-Interscience: New York, 1967; Vol. 1 pp 507, 51 1. (33) Banerjee, A.; Banerjee, G. C.; Bhattacharya, S . ; Banerjee, S.; Samaddar, H. J . IndianChem. SOC. 1981, 58,605. (34) Furniss, B. S.;Hannaford, A. J.; Robers, V.; Smith, P. W. G.; Tatchell, A. R. "Vogel's Textbook of Practical Organic Chemistry"; Longman: London, 1978; p 753.
RECEIVED for review August 26, 1985. Accepted November 4, 1985. Thanks are due to University Grants Commission, New Delhi, for a Junior Research Fellowship to A.J.
Spectrophotometric Determination of Iron in Wines, Foods, and Minerals with 5,5-Dimethyl- 1,2,3-cyclohexanetrione 1,2-Dioxime 3-Thiosemicarbazone Francisco Salinas, Juan Carlos Jimdnez SBnchez,* and Teresa Galeano Diaz Department of Analytical Chemistry, Faculty of Sciences, University of Extremadura, 06071 Badajoz, Spain
A new reagent, 5,5-dimethyl-l,2,3-cyciohexanetrione 1,2-dioxime 3-thiosemicarbarone, has been syntheslzed and studied spectrophotometricallyand a simple, rapid, selective, and sensitive method for the spectrophotometric determination of iron has been developed based upon the formation of (5,5dlmethyi-l,2,3-cyclohexanetrlone 1,2-dloxime Whiosemlcarbarone)-iron( I I ) complex. A violet color Is formed In strongly acld medium and the absorbance Is measured at 550 nm. The molar absorptivity Is 8.9 X lo3 L-mol-'.cm-' and the relative error is 0.5% (3 ppm of iron). The detection limit Is 0.05 pgmL-' of Fe(I1) and the determination limit is 0.1 pgmL-' of Fe( I I). The method has been applied to the determination of Iron in wines, minerals, and foods.
Thiosemicarbazones have been widely used for spectrophotometric determination of inorganic ions and their analytical applications have been reviewed (1-3). Thiosemicarbazones of greatest analytical interest are those that posses another complexing group. The most important of these are the N-pyridine, the oxime group, and the phenolic group, in adjacent position to the thiocarbonyl group. The other complexing group can be other thosemicarbazonic group. In this paper, the analytical properties and applications of 5,5-dimethyl-1,2,3-cyclohexanetrione1,2-dioxime 3-thiosemicarbazone (DCDT) are presented. An important section of this paper is the development of a method of determination of iron in wine. The determination
0003-2700/86/0358-0824$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
of iron in wines is a requisite routine plant control analysis. Procedures usually used for the determination of iron in wine involve "wet ashing" in the preparation of the sample for analysis. All organic matter of the wine sample should be oxidized and removed by this process. Two reaction techniques have been commonly applied in this preparation procedure. First, the nitric-perchloric acid digestion. Second, the hydrogen peroxide oxidation. In the present paper a method for determination of iron in wine is developed without the requisite wet ashing accompanying procedure in the preparation of the sample for analysis. From bibliographic data, it is deduced that other methods without this previous requisite need an ion-exchange or extraction procedure. This "in situ" procedure is thus more direct and time saving with procedural simplification and with no loss of specificity or sensitivity. Anot,her usually used procedure for the determination of iron in wine, atomic absorption spectrophotometry, needs the preparation of standards with amounts of ethanol and water similar to that of the sample. EXPERIMENTAL SECTION Apparatus. Beckman Model 25 and Zeiss DMR 11spectrophotometers with 1.0 cm path length glass or quartz cells and a Crison digit 74 pHmeter with a combined SCE-glass electrode were used. Reagents. All chemicals were of analytical reagent grade or better. Stock standard Fe(II1) and Fe(I1) solutions (1.984 gL-' and 1.002 g-L-l, respectively) were standardized, the former gravimetrically and the latter by titration with K2Cr207. All subsequent iron solutions were prepared by dilution. Soltltions of DCDT in dimethylformamide of various concentrations were used by dissolving solid reagent samples prepared and purified by the authors. Synthesis of DCDT. To obtain the DCDT, 5,5-dimethyl1,2,3-cyclohexanetrione 1,2-dioxime was previously obtained by Haas's procedure ( 4 ) modified by the authors. Two grams of was dissolved in a powdered 5,5-dimethyl-1,3-cyclohexanedione solution of 10 g of potassium hydroxide in 30 mL of water; 30 mL of ethanol was then added and, while the solution was cooled in a freezing mixture and throughly stirred, 1.2 g of KNOBwas added and the solution was neutralized with concentrated HCl until slight excess. This solution, even cold, was mixed with a solution of 1.0 g of hydroxylamine hydrochloride in water and sodium acetate was added until a pH value of 5 was obtained. The white precipitate obtained was filtered and washed with water (yield 70%). To synthesize the DCDT, a solution of 1.24g of thiosemicarbazide in 50 mL of water and 100 mL of ethanol was added to a solution of 2.5 g of 5,5-dimethyl-1,2,3-cyclohexanetrione 1,e-dioxime in ethanol. The solution was stored for 3 or 4 days and the solvent then removed at 30 "C under diminished pressure. A yellow solid is separated and recrystallizedin ethanol-water (mp 183-185 "C; yield > 50%). Anal. Calcd for CgHl5N5O2S:C, 42.02; H, 5.84, N, 27.24. Found: C, 41.54; H, 5.89; N, 26.74. A potentiometric titration confirms the purity of obtained product. Recommended Procedure for the Determination of Iron. Into a 25-mL volumetric flask, a suitable aliquot of sample solution containing up to 150 wg of iron, 3 mL of concentrated HCl, and 1 mL of 1% reagent solutions in DMF were transferred, diluted to the mark with deionized water, and mixed well. The absorbance was measured at 550 nm by using a 1.0-cm cell against deionized water. A suitable prepared calibration graph or empirical calculation was used to convert absorbance into concentration, Determination of Iron in Wines. Into a 25-mL volumetric flask, a suitable aliquot of wine containing between 2.5 and 150 wg of iron, 3 mL of concentrated HC1, and 1 mL of 1% reagent solution in DMF were transferred, diluted with deionized water, and mixed well. The absorbance was measured at 550 nm against a blank of wine with 3 mL of concentrated HC1, if measurable color was present. Determination of Iron in Vegetables and Verdures. The method has been applied to determination of iron in spinach and lentils. About 20 g of spinach, after being dried at 110 "C, was calcined and treated, heating, with 10 mL of concentrated HCl. The solution was filtered and carefully washed into a 100-mL
825
A
09-
OS
t
03;
s' d
Figure 1. Ionization constant calculation: (A) [R] = 7.78 X M, A = 285 nm. A = 315 nm; (6)[R] = 3.89 X
M,
volumetric flask. Aliquots of 5 mL were used in the spectrophotometric determination of iron with DCDT by the recommended procedure. To determine iron in lentils, about 20 g of sample was calcined and treated, heating, with 10 mL of concentrated HCl. The solution was filtered and carefully washed into a 100-mL volumetric flask and aliquots of 1 mL were used in the spectrophotometric determination of iron. Determination of Iron in Magnesites. About 0.5 g of sample was treated with 10 mL of concentrated HCl, the mixture was heated, and the solution was filtered and carefully washed into a 100-mL volumetric flask, in the event of high-purity magnesite, or into a 500-mL volumetric flask in the event of magnesitechrome. Aliquots of 5 and 1.5 mL were used, respectively,in the spectrophotometric determination of iron with DCDT by the recommended procedure. Determination of Iron in Portland Cement. About 0.25 g of sample was mixed with 0.5 g of NHICl and treated with 5 mL of concentrated HCl, and the mixture was heated in a water bath for 30 min. Twenty milliliters of hot water was added and the solution was filtered and carefully washed into a 100-mL volumetric flask. Aliquots of 1 mL were used in the spectrophotometric determination of iron. RESULTS AND DISCUSSION Analytical Properties of 5,5-Dimethyl-1,2,3-cyclohexanetrione l,2-Dioxime 3-Thiosemicarbazone. Solubility. The solubility of DCDT in several solvents at room temperature was determined by Wittemberger's method (5). DCDT is very soluble in DMF (73.2 gL-'), moderately soluble in ethanol (14.5 gL-l), isoamyl alcohol (6.2 gL-l), and methyl isobutyl ketone (6.8 gL-l), and sparingly soluble in chloroform (0.6 g G l ) and water (0.2 gL-l). Stability. Solutions of 3.8 X lom3M reagent in dimethylformamide are not very stable. It is advisable to prepare these every day. The stability of dilute aqueous solutions (3.8 X M) is smaller a t 3 < pH < 8. Spectral Characteristics. The infrared spectrum was obtained (KBr disks) and the bands were assigned (cm-l) to the stretching vibrations of C=N-OH (34201, -NH- (3280), -OH (3140), C=N- (Ego), and C=S (1130). The ultraviolet absorption spectra in water show a maximum a t 310 nm in acid medium and a maximum a t 285 nm in neutral or basic medium. Ionization Constants. The ionization constants were determined by the Stenstrom and Goldsmith method (6) a t 20 "C and ionic strength 0.1. The results obtained were pK1 = 4.41 f 0.12, pK2 = 6.26 f 0.15, and pK3 = 13.9 f 0.1 (Figure 1). pKl corresponds to the dissociation of the positively single charged reagent to give the neutral form and pK, and pK3 correspond to the successive dissociations to give the single and double charged anions, respectively. The obtained values for these dissociations constants are similar to that of other thiosemicarbazones with oxime groups (7, 8).
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
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Table I. Characteristics of DCDT Compounds ion
PH
Fe(I1) Fe(II1) Ni(I1) Ni(I1) Co(I1) Mn(I1) Cu(I1) Cu(I1)
nm
A,,
